Adaptive antennas are important subsystems for long range, mobile and ad hoc wireless communications and sensing networks. The most powerful adaptive antenna architecture is the digital beamforming array, where each antenna element has an associated signal chain to convert the received signal from RF (radio frequency) to baseband, and the baseband signals are processed by a digital beamformer using adaptive filtering techniques. Digital beamforming antenna arrays have the capability of generating many antenna patterns simultaneously, performing high precision beamforming such as nulling, and producing output signals with the maximum signal to noise and interference ratio (SNIR). A further advantage of the digital beamforming array architecture is that on-line calibration of different branches can be handled automatically by the digital beamformer. However, purely digital beamforming antenna arrays do have a major disadvantage. Since the cost of processing digital data is proportional to the bandwidth and the computational power required for digital signal processing increases at least linearly with the number of elements, a large digitally beamformed antenna array for wideband operation is simply too costly and impractical for most applications. Another problem with digital beamforming arrays is that, due to the limitation of physical space defined by the array element spacing, a pure digital beamformer is impractical at millimeter-wave frequencies beyond approximately 55 GHz. The element spacing required for suppression of grating lobes at scan angles within 60 degrees is limited to 2.9 and 1.7 mm at operating frequencies of 55 and 95 GHz respectively, thus making it extremely difficult to physically place the signal chains behind the antenna elements.
The other principal adaptive array architecture is the analogue beamformer, in which “smart beams” are formed using RF or IF phase shifters on each antenna element in the array. In purely analogue systems, cost is a very weak function of bandwidth. Thus, for wideband adaptive antenna systems with a large number of elements, an analogue adaptive antenna array is much more economical than a purely digital adaptive array. However, an analogue beamformer has certain disadvantages. First, the calibration of a large analogue array is labour-intensive and on-line recalibration is extremely difficult. Second, special means must be employed for beam tracking, thus increasing the complexity and the cost. Third, there is no access to the baseband signals of individual analogue branches because the phase-shifted IF or RF signals are combined before conversion to baseband, so the capacity to form selective beam patterns is limited. These problems are particularly severe for large analogue arrays where the imperfection and ageing of components can cause serious degradation in antenna performance.